Dimensional stabilisation by chemical modification of wood
Corsican pine (Pinus nigra) and beech (Fagus sylvatica) wood specimens were modified with n-butyl isocyanate (n-Bu-NCO), phenyl isocyanate (Ph-NCO), 1,6-diisocyanatohexane (HDI) and tolylene 2,4-diisocyanate (TDI), using pyndine as the swelling agent and catalyst. Several levels of weight gain were achieved by varying the reaction time. For each species the approximate general reaction rate order based on hydroxyl substitution was Ph-NCO > n-BuNCO > HDI > TDI for Corsican pine, and HDI > Ph-NCO > n-Bu-NCO > TDI for beech. The effect of chemical modification in the sorption properties and in the associated dimensional stability of wood was investigated by determining sorption isotherms and measuring the respective volumetric swelling at each relative humidity (RH) level. The treatment depressed the isotherms evenly over the entire range of RH for all levels of reaction, however, the difunctionally modified wood tended to show lower reduction in sorption at low RH's. The reduction in sorption was closely accompanied by a reduction in swelling indicating that within the accuracy of the experiment no voids were formed within the cell walls due to the treatment. The TDI treated specimens showed higher reduction in swelling under RH conditions than at saturation in liquid water. This was interpreted as to be caused by the formation of rigid TDI crosslinkages. The mechanisms of reduction in wood hydroscopicity were investigated by fitting data to the Hailwood-Horrobin sorption model. It was shown that the chemical modification treatment affects both monomolecular and polymolecular sorption. The accessibility of bonding sites was also reduced. Regression analysis revealed that the reduction in monomolecular sorption is chiefly governed by the decrease in site accessibility and that the reduction in polymolecular sorption is governed by the increase in the adduct volume in the cell wall. Investigation of the causes of the reduction in site accessibility by analyzing the sorption isotherms for wood modified with n-Bu-NCO and with a bulkier monofunctional isocyanate (octadecyl isocyanate) suggest that the accessibility of the internal wood surfaces is related to the weight gain and not to the fraction of hydroxyl groups (OH) reacted. This suggests that the reduction in accessibility is caused by a 'shielding effect' where the adducts physically cover some of the adjacent unreacted sorption sites. With difunctionally modified wood no evidence of crosslinking effectiveness was indicated at saturation in liquid water. However, when the samples were soaked in pyridine, the crosslinkages seemed to influence superswelling in comparison to that shown by mono functionally reacted wood. Finally, tensile strength tests carried out on modified and unmodified wood strips revealed that with the exception of n-Bu-NCO treated samples, the wood strength is slightly but significantly reduced by the treatments.